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The doctrine respecting the crystalline strata implied in the name metamorphic may properly be treated of in this place; and we must first inquire whether these rocks are really entitled to be called stratified in the strict sense of having been originally deposited as sediment from water.
The formation is upper Silurian or lower Devonian, a transition to gneiss, but not highly metamorphic. No fossils have yet been found: if any exist they would be microscopic. Where talcose it is bluish, and shows streaks of 'black sand, titaniferous iron. The grey sand washes to white. There are pot-holes which have been filled with either a pudding or a breccia of quartz.
Whether "primitive" or metamorphic, however, these Canadian rocks, and analogous ones beneath the fossiliferous strata of other countries, are the oldest portions of the earth's crust of which geology has any present knowledge.
From Parian and Pentelic marble, Lunar marble, as already mentioned, can be easily distinguished by the less brilliant sparkle of its crystal facets, as shown by a fresh surface, and also by its more soapy-white colour. It is simply an ordinary Jurassic limestone altered by subsequent metamorphic action.
Far the most beautiful and richly furnished of the mountain caves of California occur in a thick belt of metamorphic limestone that is pretty generally developed along the western flank of the Sierra from the McCloud River to the Kaweah, a distance of nearly four hundred miles.
Gneiss in geology is commonly used to designate not merely stratified and foliated rocks having the same component materials as granite or syenite, but also in a wider sense to embrace the formation with which other members of the metamorphic series, such as hornblende-schist, may alternate, and which are then considered subordinate to the true gneiss.
Having said thus much of the mineral composition of the metamorphic rocks, I may combine what remains to be said of their structure and history with an account of the opinions entertained of their probable origin.
Such clay-slates, in fact, are variable in composition, and sometimes alternate with fossiliferous strata, so that they may be said to belong almost equally to the sedimentary and metamorphic order of rocks.
Except for the occasional ridges of metamorphic rocks mentioned above, and some hills of intruded greenstone, the lower plain is stoneless, its subjacent rocks being covered with a thicker stratum of the same alluvium which is thinly spread over the higher table-land above.
Brown metamorphic slate, close-grained in places, dips away from the inlet, presenting edges to ice-action, which has given rise to a singularly beautiful and striking surface, polished and grooved and fluted. All the next day it rained. The mountains were smothered in dull-colored mist and fog, the great glacier looming through the gloomy gray fog fringes with wonderful effect.
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